I can't install an add-on (plugin)
Installation

The addon installed fine for me, no idea why you are getting that error message.

Firstly I copied the code from link provided pasted into and ran from  text editor, to test,  no issues.



Then I saved the file as blender_collections.py into my Desktop folder, hit install addon, chose the file, once again no issue



Thought how are you getting that error message, then it twigged, if you hit the download button the file is saved as blender_2.8_boolean_collections_plugin_by_ispheria.py and there is your issue.

The file name has more than one period "." in it. This is the issue.  Only have one period in the file name before the py extension.  Python uses the dot to separate modules, and objects contained therein, eg bpy.props where both are modules.

Recommend you rename the file to boolean_collections.py and it will install.

Note

Always pays to check the homepage of the addon. In this case on blender artists forum.  The same issue has been raised.